The goals of this project are to understand how the processes of proliferation and differentiation are controlled and coordinated in blood cells and how leukemic transformation perturbs these processes. Much progress has been made in identifying the transcription factors that control blood cell development, but there is still much to be learned about how they interact functionally with each other. We also know very little about how these transcription factors regulate cell proliferation and interact with the molecules that control the cell cycle. Understanding these mechanisms and relationships will lead to new insights into how cells control and coordinate the proliferation and differentiation programs. It also will provide opportunities for development of new therapeutic approaches based on the concept of stimulating leukemia cells to resume differentiation and enter terminal cell division. Work carried out during the current grant period shows that two key transcription factors, PU. 1 and GATA-1, that control blood lineage determination, mutually antagonize each other. We showed this mutual antagonism is important for normal blood cell development. We also found that PU.1-mediated repression of GATA-1 requires the corepressor and cell cycle regulator Rb. PU.1 and Rb cooperate to block erythroid differentiation in murine erythroleukemia cells. We now propose to understand the mechanism(s) by which PU.1 and Rb cooperate to repress GATA-1 and inhibit the erythroid program in immature erythroid cells and in myeloid cells. We also propose to determine the role of Rb and PU.1-mediated erythroleukemia in mice. Other studies from our laboratory indicate that PU.1 also cooperates with a cyclin D-dependent kinase, CDK6, to block erythroid differentiation and to stimulate proliferation in immature erythroid cells. We found that CDK6 can phosphorylate PU.1. We also found that, like PU.1, CDK6 can block erythroid differentiation in murine erythroleukemia cells. We now propose to understand how CDK6 phosphorylation of PU.1 controls PU.1 activity and/or metabolism. We also will test whether, like PU.1, CDK6 can promote leukemia in mice.